tracing: add saved_cmdlines file to show cached task comms

Export the cached task comms to userspace. This allows user apps to translate
the pids from a trace into their respective task command lines.

[ Impact: let userspace apps reading binary buffer know comm's of pids ]
Signed-off-by: Avadh Patel <avadh4all@gmail.com>
[ added error checking and use of buf pointer to index file_buf ]
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

kernel/trace/trace.c

@@ -2421,6 +2421,56 @@ static const struct file_operations tracing_readme_fops = {
 	.read		= tracing_readme_read,
 };
 
+static ssize_t
+tracing_saved_cmdlines_read(struct file *file, char __user *ubuf,
+				size_t cnt, loff_t *ppos)
+{
+	char *buf_comm;
+	char *file_buf;
+	char *buf;
+	int len = 0;
+	int pid;
+	int i;
+
+	file_buf = kmalloc(SAVED_CMDLINES*(16+TASK_COMM_LEN), GFP_KERNEL);
+	if (!file_buf)
+		return -ENOMEM;
+
+	buf_comm = kmalloc(TASK_COMM_LEN, GFP_KERNEL);
+	if (!buf_comm) {
+		kfree(file_buf);
+		return -ENOMEM;
+	}
+
+	buf = file_buf;
+
+	for (i = 0; i < SAVED_CMDLINES; i++) {
+		int r;
+
+		pid = map_cmdline_to_pid[i];
+		if (pid == -1 || pid == NO_CMDLINE_MAP)
+			continue;
+
+		trace_find_cmdline(pid, buf_comm);
+		r = sprintf(buf, "%d %s\n", pid, buf_comm);
+		buf += r;
+		len += r;
+	}
+
+	len = simple_read_from_buffer(ubuf, cnt, ppos,
+				      file_buf, len);
+
+	kfree(file_buf);
+	kfree(buf_comm);
+
+	return len;
+}
+
+static const struct file_operations tracing_saved_cmdlines_fops = {
+    .open       = tracing_open_generic,
+    .read       = tracing_saved_cmdlines_read,
+};
+
 static ssize_t
 tracing_ctrl_read(struct file *filp, char __user *ubuf,
 		  size_t cnt, loff_t *ppos)
@@ -3973,6 +4023,9 @@ static __init int tracer_init_debugfs(void)
 	trace_create_file("trace_marker", 0220, d_tracer,
 			NULL, &tracing_mark_fops);
 
+	trace_create_file("saved_cmdlines", 0444, d_tracer,
+			NULL, &tracing_saved_cmdlines_fops);
+
 #ifdef CONFIG_DYNAMIC_FTRACE
 	trace_create_file("dyn_ftrace_total_info", 0444, d_tracer,
 			&ftrace_update_tot_cnt, &tracing_dyn_info_fops);